6.2:
First Law of Thermodynamics
When gasoline fuels a car engine, the energy produced is released as heat and work.
In a car’s combustion engine, the fuel and air mixture contains chemical energy, which is a type of potential energy. This potential energy is then transformed into kinetic energy through a combustion reaction that generates heat.
The heat coming from the reaction is then converted to work by the expanding hot gases pushing on the piston, which then turns the crankshaft, ultimately bringing the car into motion.
All these energy interconversions must follow the first law of thermodynamics — energy is always conserved and cannot be created or destroyed.
To study energy changes in a system all sources of energy must be considered, including potential and kinetic energy. The total energy within a system at any given time is called the internal energy, which has the symbol capital-U, or sometimes symbolized as capital-E.
A system’s internal energy can change. The internal energy of a car with a full tank — its initial state — is different from the total energy while the car is running and is again different when the tank is empty — its final state.
Whether all of the fuel is used to drive 300 or 50 miles is irrelevant, when the tank is empty, the car reaches a new internal state. Thus, internal energy is described as a state function, which is not affected by how the system came to be in its current state.
Therefore, the change in a system’s internal energy, ΔU, is measured as the difference between the values of its final and initial states.
Because energy is conserved, the change in a system’s internal energy must be accompanied by an equal and opposite change in the energy of the surroundings.
In chemical systems, the change in internal energy is described by the difference of the reactant’s initial state and product’s final state. It is used to understand the energy flow of a system.
6.2:
First Law of Thermodynamics
Energy Conservation
Energy can be converted from one form into another, but all of the energy present before a change occurs always exists in some form after the change is completed. This observation is expressed in the law of conservation of energy: during a chemical or physical change, energy can be neither created nor destroyed, although it can be changed in form.
According to the law of conservation of matter, there is no detectable change in the total amount of matter during a chemical change. When chemical reactions occur, the energy changes are relatively modest, and the mass changes are too small to measure. Thus, the laws of conservation of matter and energy hold well. However, in nuclear reactions, the energy changes are much larger (by factors of a million or so), the mass changes are measurable, and matter-energy conversions are significant.
Energy Transfer and Internal Energy
Substances act as reservoirs of energy, meaning that energy can be added to them or removed from them. Energy is stored in a substance when the kinetic energy of its atoms or molecules is raised. The greater kinetic energy may be in the form of increased translations (travel or straight-line motions), vibrations, or rotations of the atoms or molecules. When thermal energy is lost, the intensities of these motions decrease, and the kinetic energy falls.
The total of all possible kinds of energy present in a substance is called the internal energy (U), sometimes symbolized as E.
As a system undergoes a change, its internal energy can change, and energy can be transferred from the system to the surroundings, or from the surroundings to the system. Thus, the surrounding also experiences an equal and opposite change in its energy.
Internal energy is an example of a state function (or state variable), whereas heat and work are not state functions. The value of a state function depends only on the state that a system is in, and not on how that state is reached. If a quantity is not a state function, then its value does depend on how the state is reached. An example of a state function is altitude or elevation. Standing on the summit of Mt. Kilimanjaro at an altitude of 5895 m, it does not matter how it was reached, whether someone hiked there or parachuted there. The distance traveled to the top of Kilimanjaro, however, is not a state function. One could climb to the summit by a direct route or by a more roundabout, circuitous path. Thus, the distances traveled would differ (distance is not a state function); however, the elevation reached would be the same (altitude is a state function).
This text is adapted from OpenStax Chemistry 2e, Section 5.1: Energy Basics and OpenStax Chemistry 2e, Section 5.3: Enthalpy.